The friction effects in a plate type clutch, comprising a pressure plate, a reaction plate, and a clutch wheel having friction pads which are releasably gripped between the pressure plate and reaction plate, always lead to a tendency for the pressure plate to become worn into a conical profile. To a lesser extent the same effect is also observable in the reaction plate. The result is that the area over which the friction pads are subjected to pressure between themselves and the pressure and reaction plates becomes displaced radially inwards as the clutch becomes more worn. This results in unequal wear occurring in the friction pads, with a consequent reduction in their effectiveness and even, possibly, disintegration of the latter.
In the case of certain clutch wheels, for example especially in that which is described in the specification of U.S. Pat. No. 4,260,048, the two pad elements that constitute a friction pad are mounted for relative axial movement, under the influence of resilient means disposed between them and within the limits of a predetermined relative axial displacement which is controlled by abutment means. The resilient means bias the pad elements away from each other. This arrangement is provided in order to ensure that there will be some degree of progressive engagement of the clutch. This leads to a requirement that the above mentioned resilient means between the two pad elements of each friction pad must be of relatively high stiffness. Although this arrangement may seem to reduce the effect of uneven wear discussed above by enabling the pad elements to adjust with respect to each other, in practice it is found that, because of the high stiffness of the resilient biassing means, this wear compensation is not achieved to any great extent.
Accordingly, if the pad elements are to be able to perform satisfactory relative movement, the resilient means arranged between them would have to be of relatively low stiffness.